The invention relates to a method of producing a segmented lens which is used in the manufacturing process of colour display tubes.
The invention also relates to the segmented lens itself and to the mould which is used for producing the segmented lens.
Further, the invention relates to a method of producing a screen, having a dotted pattern of apertures in a black matrix and electroluminescent material in said apertures, on a display window of a colour display tube, which method comprises, exposing a photosensitive material on the display window to light which has been emitted by a point source and has passed through the segmented lens and a mask, the segmented lens comprising facets having a diagonal, with bounderies between them, and changing the relative position of the segmented lens with respect to the display window over a first distance and in a first direction oblique to the boundaries of the facets during exposure of the photosensitive material.
The invention further relates to a screen of a colour display tube and a colour display tube provided with a screen that is produced by using said segmented lens.
A method of producing a screen for a colour display tube as mentioned in the opening paragraph is disclosed in U.S. Pat. No. 4,866,466. The method according to this specification describes an exposure process for manufacturing screens for colour display tubes. In this exposure process a segmented lens is used. The presently used segmented lens is manufactured by using a mould which contains a two-dimensional array of metal poles. The segmented lens comprises a glass carrier whichxe2x80x94in the manufacturing process of the segmented lensxe2x80x94is brought together with the metal mould, while keeping a well-defined distance to this mould. The space between the segmented lens and the mould is filled with a fluid plastic that hardens after it has been applied, for instance by exposing it to UV light. The segmented lens consisting of a glass carrier provided with a plastic layer which contains the facets, being the inclined top faces of the segments of the lens, is then removed from the mould.
The metal poles of this mould are stacked by just placing them next to each other, starting in one corner of the two dimensional array which typically consists of 21 by 17 metal poles each having a bottom surface of 8*8 mm2. This way of stacking has the disadvantage that tolerance errors in the parallellism of opposite sidesxe2x80x94which are in contact with a side of an adjacent metal polexe2x80x94of the metal poles add up, leading to obliquely positioned metal poles. As a result the facets of these metal poles will have a wrong inclination, causing landing errors. These misregistrations lead to a deteriorating performance with respect to colour purity. In this way of stacking, it is important that the metal poles are firmly pressed together, so that the friction between adjacent metal poles prevents that the metal poles can become detached from the mould when the segmented lens is removed
On the inner side of the display window, a colour display tube is provided with a screen. This screen has in general a black matrix structure having a pattern of apertures, in which apertures the electroluminescent material is provided. The structure of the black matrix in most common colour display tubes is either a dotted pattern or a line pattern. This pattern is produced by exposing a photosensitive material that is deposited on the inner side of the display window and by using an exposure system and the shadow mask, which serves as the colour selection means in colour display tubes. For exposing colour display tubes with a line pattern an exposure system with a continuous lens can be used. However, for colour display tubes with a dotted pattern, it is common use to apply a segmented lens in order to have enough degrees of freedom to obtain a dotted pattern on the screen that fulfils the requirements regarding good landing properties. Landing in a colour display tube is the quality that defines how well the electron beams that hit the screen coincide with the corresponding electroluminescent material.
After the black matrix layer has been applied on the inner side of the display window, another photosensitive process is used for applying the electroluminescent materialxe2x80x94for instance, three colours of phosphor like red, green and bluexe2x80x94to the areas of the display window that were left free by the black matrix structure.
In producing a screen with a dotted pattern, light from a point source is directed through the segmented lens and the shadow mask. This segmented lens comprises a two dimensional array of differently inclined facets. In case the screen is illuminated through a stationary segmented lens, the images of consecutive facets will not fit as consecutive areas on the screen. This will cause dark and light lines, during the exposure process, in the areas where the images of two consecutive facets are disjunct or overlap, respectively. This phenomenon is normally referred to as facet marking. In order to obtain a substantially uniform illumination over the entire screen, the segmented lens is moved during the exposure process. These movements are in general in oblique directions, called the wobble and drift direction, with respect to the rectangular array of facets. The wobble and drift directions are mutually nearly orthogonal. By this method, the image of one facet is by this method spread over a larger area, so that the light and dark lines are smeared out to such an extent that facet marking is reduced.
The ever-increasing demand for improved image quality in colour display tubes also leads to higher requirements with respect to the colour purity. This colour purity is directly related to the landing properties of the colour display tube and so, also to the quality of the segmented lens.
It is an object of the invention to provide a method of producing a segmented lens for the exposure system used in the manufacturing process of colour display tubes that leads to a colour display tube with an improved behaviour with respect to landing and colour purity.
According to the present invention, this object is achieved by means of a method of producing a segmented lens, in which a fluid plastic is applied between a glass carrier and a mould comprising a two-dimensional array of metal poles, each metal pole having a top section which is rectangular in shape, a top surface with a chosen inclination and a bottom section which is cylindrical in shape for engaging a corresponding two-dimensional array of round apertures in a carrier plate, which fluid plastic is hardened after which the segmented lens, being the assembly of the glass carrier and the plastic top layer having a two dimensional array of facets, is removed from the mould.
The invention is based on the recognition that the tolerances that occur in stacking the two-dimensional array of metal poles can be strongly diminished when the metal poles are supported by a carrier plate only and do not lean against adjacent metal poles. The invention is realized by making a mould which has a carrier plate that serves as the bottom. In this carrier plate a two dimensional array of round apertures is provided. The metal poles have a bottom section that is round in shape to engage the apertures in the carrier plate. By inserting the metal poles in the round apertures of the carrier plate, a two-dimensional array of metal poles is formed which are freely positioned without leading to wrong inclinations of the facets due to adding the errors in parallellism of the metal poles. In order to be able to remove the segmented lens from the mould after the plastic has hardened, it is required to fix the metal poles firmly to the carrier plate, for instance by locking them with a screw at the bottom side. Application of this invention in the production process of colour display tubes leads to a strongly improved landing performance, and so, to a better colour purity.
In EP-0737996-A1 a mould is disclosed that is in one piece. This mould has some disadvantages with respect to the mould according to the present invention. In a one-piece mould it is very difficult to have all the facets and side faces well defined and to have sharp edges between the side faces and the facets. By making a mould from separate metal poles, the transition between two adjacent facets in the segmented lens is more accurately determined because the metal poles intrinsically have side faces that are perpendicular to the carrier plate and the top surfaces, and the facets can be made with high accuracy. Another advantage of the mould according to the present invention is the possibility to replace individual poles, which may be necessary when one of the facets is damaged or in case of a manufacturing error.
In a preferred embodiment, the method of producing a segmented lens is characterized in that the dimensions of the top section of the metal poles in a plane parallel to the carrier plate are 4 mm by 4 mm.
This method results in a segmented lens with facets that are 4*4 mm2, which is in both directions half the size of the prior art facets. Using such a segmented lens with four times the number of facets enables a better design of the inclinations of the facets. The differences between the inclinations of adjacent facets will be smaller than in a segmented lens of the prior art. This is an important measure in counteracting facet marking. Further, the landing performance will be improved, because the inclinations of the facets are better defined.
A further embodiment is characterized in that the two dimensional array contains 42 metal poles by 34 metal poles.
In the prior art, a segmented lens with an array of 21 by 17 facets is used; by doubling the number of facets in both directions when the dimensions are halved, the overall size of the segmented lens is kept unaltered. This has the advantage that the equipment in the exposure system does not have to be changed and the same amount of light is available for exposing the screen.
It is another object of the invention to provide a method of producing a screen for use in a colour display tube which will result in a colour display tube with an improved performance with respect to facet marking and colour purity.
According to the present invention, this object is achieved by means of a method of producing a screen, which is characterized in that an image of a first facet in the one extreme position substantially coincides with an image of a second facet which is separated from the first facet in a diagonal direction by a distance which is a multiple, being at least two, of the diagonal of the first facet.
This is based on the insight that by wobbling the segmented lens over a multiple, of at least two, of the diagonal distance of a facet, the facet marking is reduced. This effect is reached by spreading out the exposure of one facet over a larger area of the screen.
In a further embodiment, a method of producing a screen which further comprises a second direction, substantially perpendicular to said first direction, and a second distance over which the relative position of the segmented lens is changed with respect to the display window, is characterized in that the second distance is a multiple, being at least two, of half the diagonal of a facet.
This embodiment reduces the facet marking even more as a result of the fact that it also increases the drift distance to a multiple of the distance of half the diagonal of a facet. This results in an even stronger spread of the image of one facet over the screen.
A further embodiment is characterized in that the first distance over which the position of the segmented lens is changed with respect to the display window, is two times the second distance over which the position of the segmented lens is changed with respect to the display window.
In this embodiment the wobble distance is twice the drift distance and both are a multiple of the prior art distance, thereby improving facet marking.
A preferred embodiment is characterized in that the first distance over which the position of the segmented lens is changed with respect to the display window is two times the diagonal of a facet.
By doubling the wobble and drift distance, facet marking is considerably reduced. When this double wobble and drift distance is combined with a segmented lens in which the dimensions of the facets have been halved, the absolute wobble and drift distances are not changed. This measure only requires an adaptation of the segmented lens, while the rest of the exposure system can be left the same.
The invention also relates to a screen for a colour display tube that is produced by the method according to the invention, as well as to a colour display tube provided with such a screen.
These and other aspects of the invention will be apparent from and elucidated by way of non-limitative examples with reference to the drawings and the embodiments described hereinafter.